Currently there exist two primary types of wind turbine systems: horizontal axis systems and vertical axis systems. In a horizontal axis system, the blades rotate about an axis that is in line with approaching wind. In contrast, in a vertical axis system, the blades rotate about an axis that is substantially perpendicular to the approaching wind. In both types of wind turbine systems, stress concentrations applied at a blade root and central hub can limit the potential size of a wind turbine system.
Wind turbines have become an increasingly popular apparatus for generating electricity, as they are “clean” in that they do not require consumption of finite natural resources to generate electricity and do not emit environmentally damaging byproducts. Wind turbines utilize blades that transfer the kinetic energy of the wind into rotational mechanical energy which is then converted into electricity by coupled generators. An amount of electricity that may be generated by a wind turbine is, accordingly, a function of the cross sectional “sweep area” covered by the rotating blades perpendicular to the approaching wind. Stress limits on physical components used in wind turbine apparatuses, however, currently limit the energy capacity of systems that can be constructed. For example, in a horizontal axis wind turbine system, centrifugal forces acting on a turbine blade combined with resistance to gravity as passing the closest position to earth generate a large amount of cyclic stress at the blade root and central hub. An additional amount of cyclic stress on the rotor blade root is caused by a cyclic pressure fluctuation between the blade and a turbine mast as the blade is rotating. Resonant oscillations are also possible if exciting and structural frequencies coincide.
In addition, another important criterion in evaluating wind energy systems is the power produced on a per-unit cost basis. The cost of constructing a wind turbine system is directly related to the mass of the wind turbine system—accordingly, a significant reduction in mass will result in a significant reduction in cost. As noted above, however, the sweep area of the turbine blades determines the potential amount of energy that can be generated. Therefore, in wind turbine system design, attempts are made to reduce the mass of a system without reducing the sweep area of the turbine blades. In addition, structural integrity of wind turbine systems cannot be compromised in the attempt to reduce mass. Survival of wind turbine systems in severe storms is a key design consideration.